The present invention relates to electroabsorption (EA) optical modulators and, more particularly, to an impedance matching circuit for EA modulators that provides broadband matching while being physically separated from the location of the EA modulator.
In a conventional arrangement of an electroabsorption optical modulator, the modulator is positioned on an optical substrate, with an input (cw) optical signal applied along the input facet of the optical device and an output, modulated optical signal exiting from the output facet of the optical device; the input and output facets being defined as a pair of parallel endfaces. An electrical modulating signal is coupled to a surface area of the electroabsorption optical modulator, where the presence of this electrical signal will alter the characteristics of the input optical signal so as to produce a desired modulated waveform in the output optical signal.
In most cases, a microstrip transmission line element is used to couple the electrical signal between an external signal source and the electroabsorption optical modulator, due to the high frequency of the modulation signal. In order to allow for optimum signal transfer from the external signal source to the optical modulator, it is beneficial to provide impedance matching between these elements to improve the return loss of the modulator (hence, providing improved efficiency in the optical system). In most cases, a 50-ohm terminating resistor (i.e., transmission line) is connected in parallel with the modulator and used for impedance matching purposes. If the modulator is operating at high impedance, then the transmission line provides a good match and power is conserved. However, if the modulator is operating at a low impedance level, the impedance as seen from the electrical signal generator declines, impedance mis-matching occurs and the optical output waveform deteriorates.
Satisfying the desired input impedance matching specifications for an electroabsorption modulator in both low frequency and high frequency modes is problematic in that the impedance matching circuit needs to be electrically located near the modulator (to reduce the value of the transmission line impedance between the circuit and the modulator), while from a physical standpoint, it is desirable for the impedance matching circuit to be formed as a separate component, in order to optimize produce manufacturability.
A need remains in the art, therefore, for an impedance matching circuit that meets both the desired electrical and physical criteria.
The need remaining in the art is addressed by the present invention, which relates to electroabsorption optical modulators and, more particularly, to an impedance matching circuit for EA optical modulators that provides sufficient broadband matching while being physically removed from the location of the EA optical modulator.
In accordance with the present invention, an impedance matching circuit is formed to include both a low frequency matching section and a high frequency matching section, and configured into a topology such that a series inductor (used for low frequency impedance matching) is disposed between the signal source/high impedance matching circuit and the EA modulator itself. The use of a series inductor, therefore, allows for the high frequency portion of the impedance matching circuit to be physically separated from the modulator (i.e., formed off-chip with respect to the modulator). A conventional 50-ohm resistance element (such as a transmission line) may be used to provide high frequency impedance matching, as with the prior art.
In a preferred embodiment of the present invention, the series inductor comprises a set of wirebonds disposed between the transmission line and the modulator, although other arrangements are possible. Indeed, other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.